Insights into the use of phenylalanine ammonia-lyase as a marker for the selection of olive cultivars with improved virgin olive oil functionality

Virgin olive oil (VOO) represents the main source of lipids in the Mediterranean diet, its phenolic fraction being one of the main contributors to the health-promoting properties of this diet. Phenolic composition of VOO is directly related to the content of phenolic glycosides present in the olive fruit, which undergo a series of modifications during the oil extraction process. The vast majority of these phenolic glycosides comprise phenylethanoid derivatives of tyrosol and hydroxytyrosol whose metabolic origin are the hydroxylated aromatic amino acids 4-hydroxyphenylalanine (tyrosine) and 3,4-dihydroxyphenylalanine (L-DOPA). However, the synthesis of phenolic compounds in plants has traditionally been linked to the action of the phenylalanine ammonia-lyase (PAL) enzymes on the non-hydroxylated aromatic amino acid phenylalanine, thus constituting the first step in the biosynthetic pathway of different families of phenolic compounds known as phenylpropanoids. In this research, the implication of PAL in the conformation of the phenolic profile of olive fruit and VOO has been studied through the combined analysis of phenolic composition, PAL activity and gene expression in the mesocarp of the fruit from olive cultivars whose oils have very contrasting phenolic contents. According to the data, the role of PAL would be restricted to the most advanced stages of the fruit ripening process, controlling the channeling of phenylalanine towards the synthesis of phenylpropanoids, mainly anthocyanins, and thus reducing the metabolisation of tyrosine and L-DOPA towards the synthesis of phenylethanoids. Consequently, it is advisable not to use the selection of olive cultivars based on molecular markers related to PAL genes in olive breeding programs aimed at obtaining cultivars that give rise to oils with improved functional properties.